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This is the accepted version of a paper presented at Conference on Electrical insulation and Dielectric
phenomena, CEIDP 2015.
Citation for the original published paper:
Ariza, D., Becerra, M., Hollertz, R., Pitois, C. (2015)
Propagation of negative streamers along mineral oil-solid interfaces.
In: Proceedings of the 2015 CEIDP Conference on Electrical insulation and Dielectric phenomena,
CEIDP 2015 IEEE
IEEE Conference on Electrical insulation and Dielectric phenomena
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
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Propagation of negative streamers along mineral oilsolid interfaces
David Ariza1, Marley Becerra1,3, Rebecca Hollertz2, Claire Pitois3
1
Dept. of Electromagnetic Engineering, School of Electrical Engineering, KTH, Royal Institute of Technology
Division of Fibre Technology, School of Chemical Science and Engineering, KTH, Royal Institute of Technology
3
ABB Corporate Research
Stockholm, Sweden
dfag@kth.se, marley@kth.se, rhollert@kth.se, claire.pitois@se.abb.com
2
Abstract— This paper introduces an experimental study on
the propagation of negative streamers along mineral oil-solid
interfaces. A standard type of impregnated paper and different
polymeric films (made of PET, PTFE and PVDF) are selected as
solid materials immersed in mineral oil. The effect of the solid
material on the streamer propagation along the interface formed
with transformer oil is studied. Streamer velocities classified as
first mode propagation point cathode are reported. Voltage
gradient of the streamer channel and its stopping voltage are
calculated for all the cases. Comparison of streamer charge and
stopping length propagation are reported.
Keywords—Streamers,
impregnated paper, .
creeping
discharges,
mineral
Fig. 1 Sketch of the experimental setup. (1) Differential amplifier, (2) high speed camera,
(3) test chamber, (4) probe-plane and point–plane configuration, (5) fiber optic, (6)
photomultiplier, (7) high voltage pulse source, (8) oscilloscope, (9) xenon lamp power
source, (10) xenon lamp, (11) signal generator, (12) USB data acquisition device, (13)
computer, (14) and (17) valves, (15) magnetic pump, (16) particles filter, (18) heating rod
oil,
I. INTRODUCTION
The development of tailor-made materials provides new
possibilities to improve the dielectric performance of electrical
insulations systems. Unfortunately, there is a lack of
understanding about which characteristics need to be modified
in order to make a material suitable for electrical insulation
applications. The analysis of the streamer propagation at
liquid-solid interface has a relevant importance for the
understanding of the streamer properties [1,7,8]. The present
work is intended to present the results of streamers propagating
along the interface of different solid materials with mineral oil.
The comparison of these results contributes to the
understanding of the materials properties that affects the
streamer propagation; namely voltage gradient of the streamer
channel, stopping length, velocity, total charge accumulation
and others. In this study an impregnated paper and three
polymeric films are selected to compare the influence of the
material permittivity in the streamer propagation.
II. EXPERIMENTAL SETUP
Figure 1 introduces the complete experimental setup to
study the prebreakdown processes in mineral oil in the
presence of solid interfaces under a needle-plane configuration.
It consists of a vertical needle with tip radius of 2.9 µm and
gap distance of 5 mm to the plane electrode. The volume of the
test chamber is 6 × 10−3 𝑚3 and it is half-filled with mineral
oil. The solid material is in an inclined position (30 degrees)
respect to plane (see section II.D). Square positive voltage pulses
are applied to the plane electrode.
This work is sponsored by ABB corporate research, the Swedish Centre for
Smart Grids and Energy Storage SweGRIDS, the Swedish strategic research
program StandUp for Energy and the EIT Innoenergy Materials platform.
A. Measurements
The streamer propagation is studied based on the following
measurements:
•
•
•
Injected charge
Light emission
Shadowgraphs
The streamer charge measuring system consists of an
integrating circuit with a known capacitor connected to the
needle electrode to integrate the streamer current. When the
applied voltage rises, a large displacement current circulate in
the needle electrode superposed to the streamer current and
resulting in an undesired voltage level of the current
integration. The problem is avoided by using a differential
measurement, based on an additional blunt probe electrode to
integrate the displacement current [2]. Then a differential
amplifier is used to subtract the signals from the needle
electrode and probe electrode signals. A full description of the
measuring system is presented in [1].
The streamer light emission measuring system consists of
an optical fiber installed 5 mm away from the point electrode
and facing the needle tip. The optical fiber drives the light
signal outside of the test chamber until the photomultiplier
sensor, Figure 1.
In addition, a single frame shadowgraph of each streamer is
obtained by aligning a light beam from a Xenon lamp, the
needle tip and the camera (with an accuracy of ±0.15𝑚𝑚 )
through a set of lenses L1, L2, L3, L4. The optical arrangement
is focused to detect a 730 × 730 𝜇𝜇2 image close to the
needle tip (20x magnification).
B. Transformer oil preparation
The test chamber is capable to degas and filter the oil
pumping it thought a filter with pore size of 2 µm . A
continuous flow of oil thought the chamber, pump and the filter
is kept on during 24 h at 60 °C and 6 mbar. Once the filtering
and degassing process is finished, the chamber is cooled down
to room temperature (≈23 °C) and filled with dry air up to
atmospheric pressure. All tests are performed at atmospheric
pressure.
C. Solid sample preparation and assembly
Different polymers are chosen for the solid interface in
addition to the impregnated paper, which is the reference
material in the insulation system of power transformers (Table
I). Polyethylene terephthalate PET (C10H8O4)n is selected as a
polymer with the same permittivity as the impregnated
pressboard but with different surface morphology and chemical
composition. Polytetrafluoroethylene PTFE (C2F4)n is chosen as
a material to match the permittivity of the oil at the interface.
Polyvinylidene fluoride PVDF (C2H2F2)n is tested as a material
with a large permittivity mismatching with transformer oil. The
tested paper or films are cut into long narrow pieces of
8 × 100 𝑚𝑚2 .
III. PROCEDURE
Series of square high voltage pulses with 35 ns in rise time and
50 µs width are applied to the plane electrode. The multi-level test
method with voltage levels ranging between 11.5 and 22 kV and
steps of about 0.6 kV is used. In order to detect changes in the
solid samples or the oil during the experiment, all the tests for each
level are partitioned into four different randomized series of 10
shots each. All the results presented at each level correspond to the
average of 40 measurements. The waiting time between voltage
shots is 1 minute.
IV. RESULTS AND DISCUSSION
Figure 3 presents a typical oscillogram recorded for a
streamer propagating in mineral oil without any solid (the
oscillograms for the cases of oil-solid interface are very similar
in shape). Curve a corresponds to the exposure time of the high
speed camera recording the picture up to the last charge
injection step. Curve b is the streamer light emission recorded
with the photomultiplier (in arbitrary units). Notice that this
signal presents a continuous component that is typical of the
light emission at the head of negative streamers propagating in
oil [11]. The streamer charge is shown in curve c, which shows
the typical propagating steps of negative streamers. These steps
are strongly correlated with the light pulses [7]. The magnitude
and frequency of the light pulses increase with voltage. These
pulses appear after a short period (about 2 µs) after the start of
the voltage.
Fig. 3 Oscillogram of streamer propagating in transformer oil
TABLE 1. SAMPLES CASES
Impregnated
paper
PET
PTFE
PVDF
Oil
Thickness
(mm)
Permittivity
0.17
3.0
0.1
0.1
0.1
-
3.0
2.0-2.1
8.4
2.2
Density
(𝒈/𝒄𝒄𝟑 )
0.7
1.3
2.2
1.76
0.87
All samples are preheated at 120°C for 24 h in order to
remove all the absorbed moisture and then impregnated with
filtered and degassed transformer oil at 60°C and 6 mbar for 24
h. The impregnation process is presented in [1]. The solid piece
is installed on a holder mounted on two precision mechanical
stages that allow rotating the strip piece with a precision of 2.4
arcmin per revolution of the micrometrical screw control and it
moves along an axis direction with a precision of 0.5 mm per
revolution of the micrometrical screw control. This assembly
permits to install the strip piece sample touching the point
electrode tip (as shown in Fig. 2).
Fig. 2 Assembly of the impregnated paper in inclined position
Figure 4 shows a typical example of the frame images
recorded with the camera a) when the streamer is propagating
along the oil-solid interface and b) in the next frame 25 s after
the voltage has dropped (no streamer). The shadowgraph of the
streamer is then intensified by making the digital difference
and filtering between these two frames (Fig. 4.c).
Fig. 4 Example of the frames recorded for a streamer propagating along the
mineral oil-PET interface. a) frame taken during the streamer propagation . b)
following frame 25 s after the voltage is dropped and the streamer has
collapsed. c) The digital difference between the frames a) and b).
Typical shadowgraphs of the propagation of streamer for
negative polarity along the oil-solid interfaces studied are
shown in Fig. 5. These time-integrated shadowgraphs show the
streamers at their stopping length. The structure of the streamer
is in general a single filament for the cases of the oil-solid
interfaces and an ionized gas cavity of irregular shape for the
case in oil. Differences of the morphology of the filament
streamer in thickness and length are appreciable for the
different tested interfaces. Notice that the pictures presented in
Fig. 5 have two parts. First part show clearly where the solid
interface edge starts and second part (filtered picture) shows a
detail of the streamer edge in contact with the solid interface.
the same. Nevertheless it is not possible to conclude that this
voltage condition is independent of the oil-solid interface case
because PTFE presents a different value.
Fig. 6 Average streamer length for point cathode at transformer oil and
transformer oil-solid interface for materials presented in section III.
Fig. 5 Shadowgraphs of streamers propagating along the different solid-liquid
interfaces at 21kV. a) Transformer oil-PTFE interface, b) Transformer oilPET interface, c) Transformer oil without solid interface, d) Transformer oilPVDF interface and e) Transformer oil-Pressboard interface
Figure 6 presents the streamer stopping length estimated from
the shadowgraphs as a function of the applied voltage. A linear
correlation is observed for all the considered cases. This
indicates a constant potential gradient along the channel of the
streamer as proposed in [7]. The mean value of the potential
gradient (E) calculated as the slope of the best fitting lines of
the experimental data is reported in the legends for each curve.
Interestingly, this mean potential gradient is similar for all the
polymers considered, despite of their large differences in
permittivity. This results shows that the permittivity of a
polymeric interface does not have a significant influence in the
potential gradient in the streamer channel. For the case with the
impregnated paper interface (with the same composite
permittivity as PET), the potential gradient is about two times
larger. Assuming that the permittivity of the impregnated paper
does not affect the streamer potential gradient either, this
difference can be attributed to the different surface morphology
and chemistry compared with the polymeric films. For
streamers propagating in oil without interface, the potential
gradient is slightly larger than with the pressboard interface.
According with [8] streamer propagation stops when the
electrical potential of the channel tip reaches a specific value. It
is possible to estimate the stopping voltage value from the
intersections of the curves with the voltage axis in Figure 6. It
is circa 14.7 kV for PTFE and about 12.2 kV for the other
cases. It is interesting that even when the streamer is
propagating along surfaces with different permittivity and
chemistry, the stopping voltage condition of the channel tip is
Figure 7 presents the streamer velocity as a function of the
applied voltage for all the cases. The velocity reported is
calculated with the maximum streamer length measured from
the pictures and the time when the last streamer charge
injection is detected. All velocities reported range between 10
and 280 m/s. These streamers can be classified as slow
streamers according with [9,10]. These are first mode
propagation point cathode streamers, typically observed in
short gaps at low voltage and tip radius around 1 µm. Those
streamers consist of vaporized liquid with low conductivity.
Notice that all the velocity curves have two major parts. A first
part where the velocity increases linearly with the voltage and a
second part with a rather constant velocity (this assumption is
valid only for the range of voltage applied. For much higher
voltages an increment in the velocity and changes of mode
propagation is expected. It is possible to estimate that the
transition between the linear and the constant part of the
velocities curves occurs when the streamer length is about 100
µm for all the polymers cases and about 75 µm for the case of
the impregnated paper and without interface.
Fig. 7 Average streamer velocity for point cathode at transformer oil and
transformer oil-solid interface for materials presented in section III.
Figure 8 presents the final streamers charge as a function of the
applied voltage. The increment of the streamer charge with the
applied voltage presents an exponential correlation. This result
is similar to the streamer charge and voltage correlation
reported in [7]. Notice that the total amount of charge is
different for each case and the charge tends to increase with the
permittivity of the oil-solid interface case. The streamer
channel presents a voltage gradient, but still can be considered
rather conductive. According with [7] the electric charges exist
at the interface of the gaseous channel and the liquid. In this
case a large part of the charges distributed along the streamer
channel is in contact with the solid, see Figure 5. It increases
the possibility of accumulation and deposition of charges on
the interface.
Fig. 8 Streamer charge average for point cathode at transformer oil and
transformer oil-solid interface for materials presented in section III.
V. CONCLUSIONS
The creeping discharges propagating along different solid
interfaces reported in this study are classified as first mode
propagation point cathode streamers. All the streamers
propagate in mineral oil but in contact with the solid interface.
The voltage gradient of the streamer channel and its stopping
voltage are calculated for all the cases. The estimated potential
gradient of the streamer channel has been found to be weakly
influenced by the permittivity of the solid material. The
stopping voltage condition of the channel tip is the same for the
cases in without interface and with impregnated paper, PET
and PVDF interfaces. However, it is different for the PTFE
interface. The total streamer charge increases for all the cases
when the applied voltage increases. A nonlinear correlation
between the charge and the length of the streamers is found,
which is typical of the streamers propagating short distances (≤
400 µm) from the point electrode tip. The total injected charge
is found to increase with the permittivity of the solid interface.
REFERENCES
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Figure 9 presents the streamer charge as a function of the
stopping length. Notice the strong quadratic correlation for all
the cases. It is important to highlight that this nonlinear
dependency is typical for negative first mode streamers. In
these conditions the streamer propagates in the range of 0-400
µm from the tip electrode with a slow velocity and the streamer
charge ranges between 0-300 pC. This nonlinear dependency
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It is interesting to observe that Figure 9 shows that the
streamer charge per unit length depends of the type of oil-solid
interface. The cases of oil impregnated paper and transformer
oil without interface present similar values. The polymeric
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interface is in between these two groups. . Notice that a large
permittivity of the solid material (as in the case of PVDF) does
not result in a drastic change in the streamer charge per unit
length.
Fig. 9 Streamer charge as a function of the streamer length for point cathode in
transformer oil with and without solid interface.
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